When the CPU needs to access data and programs from the hard drive, it needs to be copied into the RAM first.

Swap space is memory (not storage) in Linux allocated for overflow when the RAM is full. Older pages will be swapped into the swap space so that the RAM is freed up to continue running other tasks.

Recommended swap space size is double for small RAMs (2GB) and equal size for mid RAMs (8GB).

The total RAM and swap space is called virtual memory.

Ref

In Windows, this is known as the swap file.